Modern cameras frequently include optical systems, sometimes called zoom lenses, having multiple or infinitely variable focal lengths ranging from wide-angle to telephoto. It is common in such optical systems to include lens groups that are moveable relative to each other along the optical axis to change the focal length of the system. When the focal length is changed, it is desirable at the same time to maintain focus throughout the entire range of focal lengths. This adjustment usually is accomplished by moving the optical elements relative to the film or other imaging plane.
In some lenses, an internal lens element or group is moved to change the lens focus. This is usually accomplished by the internal group changing the focal length of the lens unit slightly, thus moving the image plane and changing the back focus. This is called "internal focusing," and, since the focusing motion is internal to the lens, the lens unit itself does not move. Although compact, internal focusing requires separate focusing and zooming motors.
There are many techniques for controlling the movement of optical elements in a zoom lens system to maintain focus while also providing for multiple or infinitely variable focal lengths. One such technique is illustrated in U.S. Pat. No. 4,971,427, filed in the name of Takamara et al. and issued Nov. 20, 1990. The Takamara et al. patent discloses a camera having front and back lens groups and a bell crank coupled therebetween to vary the inter-group spacing and thereby change the focal length. Both lens groups are retained in a lens barrel that is movable along an optical axis relative to the film plane. Movement of the lens barrel causes the bell crank to engage and follow a cam plate that rotates the bell crank and shifts the back lens group along the optical axis relative to the front lens group.
Another technique is disclosed in U.S. Pat. No. 4,391,496, filed in the name of Schilling et al. and issued Jul. 5, 1983. According to the Shilling disclosure, an elbow linkage couples first and second lens groups in a pancratic objective system. The linkage includes a cam follower at the elbow for engaging and following a cam surface so that axial movement of the first lens group extends or contracts the linkage, changing the distance between the lens groups and the system focal length.
Optical systems that simultaneously change the focus and focal length, and particularly zoom lenses that include a cam and follower mechanism, such as those described above, are particularly sensitive to design and manufacturing tolerances. Axial movement of the lens groups, intended to focus the system, simultaneously is translated by the cam and follower mechanism into relative displacement of the lens groups, changing the focal length. In automated cameras that include switches and motors for driving the focusing mechanism, relatively precise alignment and tolerances are required, not only in the cam and follower mechanism, but throughout the position sensing and switching actuators.
In addition to precision requirements, prior art mechanisms frequently are complicated and typically provide for only one focus position at each discrete focal length, either increasing cost or reducing flexibility.
My commonly assigned U.S. Pat. No. 5,333,024, which issued on Jul. 26, 1994, discloses an optical apparatus that solves these problems by having first and second lens groups moveable along an optical axis, and including a cam and follower mechanism for changing the focal length of the optical system by communicating relative axial displacement to the second lens group in response to axial movement of the first lens group. The cam and follower mechanism is characterized by a plurality of cam steps, each having (1) a dwell cam surface extending parallel to the optical axis for accommodating axial movement of the first lens group to permit focusing of the optical system between focus positions at a fixed focal length for maintaining the relative axial displacement constant in response to axial movement of the first lens group, and (2) a ramped cam surface extending at an angle to the optical axis for displacing the second lens group relative to the first lens group in response to axial movement of the first lens group for changing the relative axial displacement in response to axial movement of the first lens group. The cam and follower mechanism includes a plurality of opposed pairs of cam steps, each pair including dwell cam surfaces extending parallel to each other and the optical axis, and ramped cam surfaces extending at an angle relative to each other and the optical axis.
While most zoom lenses used in compact cameras need two separate motors for zooming and focusing because the movement of the internal groups relative to the whole lens unit is different for zooming and focusing, the zoom lens mechanism of aforementioned U.S. Pat. No. 5,333,024 enables use of but a single motor for both zoom and focus. In certain parts of the cam, the lens groups move together as a unit (unit focusing). Although this is a simple way to focus, unit focusing requires a large amount of lens travel, especially at higher zoom focal lengths. This is a serious disadvantage, especially in small, compact cameras which have a large focus range. Accordingly, it would be a great improvement if a mechanism were available that reduced the focus travel significantly over unit focusing devices.